The need for differential pressures within microwave devices is well established. For various applications, including high power microwave transmission, the waveguide through which the transmission takes place is routinely pressurized to suppress arcing within the waveguide. In other applications, including the use of microwave transmission to heat moving fluids in industrial and food processing processes, pressurized inert gas may be applied to the heating chamber to prevent combustion of the elements being heated. In still other applications, certain portions of the microwave circuit may operated under a vacuum.
In response to these needs, microwave windows have been developed. These windows are designed to insulate microwave circuits from such differential pressures, but they enable the propagation of microwaves without introducing reflection or internal resonance. For example, U.S. Pat. No. 3,781,726 entitled “Wave guide Window Assembly” discloses a waveguide gas pressure window that in a single assembly provides a waveguide window and a pressure seal to gases, while at the same time providing a low loss transfer of power through the window from a first waveguide section into a second waveguide section. The waveguide window includes a plate having the shape of the waveguide flange with a seal material positioned in a groove on both sides of the plate to contact the two waveguide flanges between which the window is mounted. The flat plate has a common flanged opening formed on one side thereof in which a window structure is positioned on shoulders and firmly bonded in position with a suitable retaining material. The window structure is formed of a suitable dielectric material such as a Teflon fiberglass plate having copper sheets deposited on both sides thereof with the copper etched or removed from the fiberglass to provide a window having the desired impedance matching characteristics.
In a further example, U.S. Pat. No. 3,675,165 entitled “Wave guide Window for Transmission of Electromagnetic Waves” discloses a window for sealing a waveguide gas pressurized to transmit microwave signals. This system includes a dielectric plate permeable to the microwave signals and a mechanical structure hermetically sealing the dielectric plate in the interior of the waveguide and including four openings dimensioned to provide the window with a substantially flat voltage standing wave ratio versus frequency characteristic over a predetermined frequency range. The system further includes two relatively movable members to permit the dielectric plate to move to compensate for unequal gas pressures on opposite surfaces thereof, and a space to allow movement of the two members to compensate for thermal expansion thereof due to microwave signal loss in the dielectric plate.
In a still further example, U.S. Pat. No. 4,556,854 entitled “Microwave Window and Matching Structure” discloses a circular waveguide window between two rectangular waveguides having increased bandwidth and increased power handling capability. It uses particular window and impedance matching structures whose dimensions are related in a particular way to the dimensions of the rectangular waveguides.
While all of these windows form gas tight seals within a microwave circuit, in certain applications, it is desirable to allow gas to flow across such a boundary in order to equalize the pressure on either side of the window, or to bring a pressure differential across a window within predetermined bounds. In these situations, gas flowing within the microwave circuit could contain small particles that facilitate waveguide arcing or damage intricately machined components. One attempt to address this problem can be found in U.S. Pat. No. 5,041,804 entitled “Particle Filter for Wave guides.” In this patent, a waveguide is provided which can include a bearing supporting a probe or other equipment for rotation. In order to prevent particles of the waveguide metal or waveguide cleaning materials embedded in the metal from entering the bearing, a window is fitted across the waveguide at a selected position between two waveguide sections or at each of a number of positions, the window being transparent to microwave transmission, being capable of transmitting pressure and being capable of filtering particles of a diameter in excess in 0.005 inch. In forming a porous window for this application, however, constraints on the material and structural aspects of the window that are required for achieving the desired filtering capacity may not result in desirable electromagnetic or physical performance. In addition, trapping particles within the transmission line can cause undesirable arcing, and, should the window of this patent become clogged or damaged, the waveguide system must be disassembled in order for the filter to be cleaned or replaced.
Accordingly, a need exists for a system for controlling the flow of a gas through a microwave circuit. The system would preferably be an integrated system that could be placed in-line with a waveguide structure, result in low loss or disruption to the electromagnetic signal, and permit gas to flow through the system while stopping particulates that might cause mechanical or electrical damage to the microwave system.